Generally, a bumper structure of a vehicle includes bumper brackets mounted on each front of side members through bumper stay reinforcements. When the vehicle is involved in a front-end collision, the front side members and the bumper brackets collapse in a longitudinal direction thereof to absorb the energy of impact.
Various attempts have been made to provide lighter weight side members with good energy absorption qualities, while maintaining a low cost and ease of manufacturing. See, for example, Axial Crush and Energy Absorption Characteristics of Aluminum/GFRP hybrid Square Tubes, Journal of the KSAE (Korean Society of Automobile Engineering), Vol. 8, No. 6, December 2000. However, to date, fully satisfactory results have not been widely achieved at levels sufficient to reduce costs and ease manufacturing.